The present invention relates to a method and composition for the direct electroplating of metals onto dielectric based materials and printed circuit fabrication. During the past decade processes have evolved which enable direct electroplating of metals onto dielectric substrates for imparting electric conductivity and continuity as are required in printed circuit fabrication. One of the main advantages recognized in these new processes is the ability to eliminate the electroless plating step, such as electroless copper. The elimination of the electroless plating step, particularly copper, significantly reduces manufacturing costs as well as minimizes the negative environmental effects associated with the strong chelates commonly present in the electroless plating compositions. Though the field of direct electroplating onto dielectrics is still in its infancy stage, extensive development activities and commercial interest have lead to several key United States patents granted in the pursuit of achieving the above stated objective.
The following are a few U.S. patents which are aimed at the direct metallization of non-conductive substrates and printed circuit fabrication: Morissey et al, U.S. Pat. No. 4,683,036; Holtzman et al, U.S. Pat. No. 4,891,069; Appelt et al, U.S. Pat. No. 4,969,979; Obakayashi, U.S. Pat. No. 5,071,517. These patents are included herein by reference.
From a review of these patents it will be recognized that the following are the three generic steps which are used throughout the various processes, regardless of the type of printed circuit board or dielectric contemplated. These steps are:
1. Contacting the surface with a cleaner/conditioner to condition the surface for the reception of a noble metal catalyst. PA1 2. Contacting the conditioned substrate with a noble metal catalyst composition comprising the key ingredients of stannous metal salts and palladium metal salts which are permitted to react to provide a colloidal palladium product. PA1 3. Direct plating from a suitable electroplating bath for the deposition of a required metal onto the non-conductive substrate, particularly in the printed circuit through-holes. PA1 1. Excessive adsorption of palladium catalyst. PA1 2. The presence of chelating agents. PA1 3. Excessively high temperature of operation for the cleaner/conditioner. PA1 4. Susceptibility to "voiding" when the circuit boards are micro-etched after processing.
From a review of the above cited references and generic steps, it will be recognized that each of the steps plays a major role in the intermeshing of the overall processing. More specifically, the successful cleaning and conditioning of the substrate is a prerequisite for the successful adsorption of the catalyst, which thereby provides a uniform surface with metallic sites that constitute the initial conductivity of the surface. It is therefore mandatory to insure that all steps are tailored for the specific process, rather than randomly select compositions from the prior art.
Since the present invention is primarily concerned with the development of a suitable cleaner/conditioner composition which will lead to the best results in a direct metallization process, the following observations are noted in respect to the prior art.
U.S. Pat. No. 4,683,036 recognized the need for a cleaner/conditioner. At column 10, lines 5 through 11, a cationic surfactant is suggested along with a non-ionic surfactant of an alkanolamine derivative. However, the nature of the cationic surfactant is ambiguous and not clearly defined. No further attention is given to the cleaner/conditioner and its criticality in the process. In fact, the teachings of U.S. Pat. No. 4,683,036 dictate that certain additives must be incorporated into the electroplating bath which impose additional requirements. The other prior art references have shown some improvements which do not dictate the addition of additives into the electrolytic plating bath.
U.S. Pat. No. 4,891,069, at column 2, lines 37 and thereafter, outlines the generic steps required for processing, with emphasis on the cleaner/conditioner. The key ingredients in the cleaner/conditioner of this patent, among others, are a chelating agent and a surfactant. The nature of the surfactant is broad, and any surfactant, regardless of its class, may fit the requirement. The generic composition for the cleaner/conditioner is further discussed in column 12, with the repetition of the key component required. Once again, the nature of the surfactant is ambiguous, referring only to a member of the group consisting of non-ionic, cationic, anionic and amphoteric types. There are no teachings nor suggestions in the specific enabling examples for the use of mixtures of surfactants; however, chelating agents such as tartaric acid and citric acid, as well as others, are stipulated.
U.S. Pat. No. 4,969,979 demonstrates a departure from other prior art by the utilization of a polyelectrolyte surfactant in the conditioning of the substrate prior to catalyzation. The patent broadly claims the treatment of the substrate by a surfactant solution. U.S. Pat. No. 4,969,979 appears to be in conflict with some of the prior art. Although the '979 patent relates to direct electroplating, the invention appears to rely on the Lindsay U.S. Pat. No. 4,701,350 which demonstrates and claims improved reliability associated with the use of polyelectrolytes in printed circuit through hole metallization.
U.S. Pat. No. 5,071,517 recognizes the need for the cleaner/conditioner in achieving positive results from the process. Though significant effort was documented, particularly in the synthesis of a colloidal noble metal dispersion, the cleaner/conditioner employed is a proprietary product sold by the Shipley Company as "Shipley 1175 Cleaner/Conditioner". In the process this product is used at 180.degree. F. for best results, although the product bulletin recommends a temperature of only 135.degree. F. The 1175 product also contains chelating agents such as triethanolamine and monoethanolamine, as revealed in the product's Material Safety Data Sheet.
In summary, certain drawbacks appear to exist in the cleaner/conditioners of the prior art:
The overall objective of the present invention is to provide a composition of matter for the cleaner/conditioner and a process for direct metallization, wherein the cleaner/conditioner will be a product that can be used at moderate temperatures, provide good catalyst adsorption to the substrate, eliminate the use of chelating agents, and provide both cleaning and conditioning in one step, in either an alkaline or acid medium.